The present invention relates to a method of driving a liquid crystal display device which employs a phase transition liquid crystal.
In typical liquid crystal display devices, a pair of glass plates are spaced apart by a spacer to form therebetween a gap of tens of microns, into which gap a liquid crystal is introduced. Between the glass plates are provided a pair of electrodes each having a desired display pattern, the upper one of which is formed of a transparent conductive film while the lower electrode is formed of a transparent conductive film or metal film depending upon how the devices are employed. In some of such devices, a phase transition liquid crystal, for example, a nematic-cholesteric phase transition liquid crystal is employed. Such a phase transition liquid crystal exhibits an effect between the peak value or root mean square value of the applied voltage and the relative brightness which is similar to a hysteresis effect. This effect is hereinafter referred to as the holding effect. When a liquid crystal, which has been applied with a holding voltage and is made low in brightness, is next applied with a write-in voltage to obtain a high brightness and then applied again with the holding voltage, the liquid crystal can maintain the high brightness due to the above-mentioned holding effect. In the past there have been known various driving methods in which a holding voltage is applied to a liquid crystal in order to enhance the speed of phase transition or to prolong the memory time (or the holding time).
A typical example of such driving methods is disclosed in, for example, an article entitled "Pulse-Length Modulation Achieves Two-Phase Writing in Matrix-Addressed Liquid-Crystal Information Displays", by Karl Heinz Walter and Miroslav Karl Taner, IEEE Transactions on Electron Devices, Vol. ED-25, No. 2, February 1978, pp 172 to 174. In this method, a liquid crystal display device is employed which includes upper and lower (X- and Y-) electrodes arranged in the form of a matrix, wherein selected ones of the upper and lower elecctrodes are applied with voltages so that a liquid crystal picture element defined by a cross point of the selected upper and lower electrodes is applied with an electric field in the direction of thickness, and a desired numeral, character, or picture image is displayed by a plurality of such picture elements.
Further, this driving method is based upon an effect similar to the hysteresis effect between the intensity of electric field established across a liquid crystal layer (or the peak value of applied voltage) and the relative brightness, and therefore applies to the liquid crystal layer an erasing, write-in, or holding voltage which is formed by changing the peak value of pulse applied to each electrode, to drive the device in a time-divisional fashion. However, in this method, in order to prevent the flicker on a liquid crystal display plane, it is required to employ a frame frequency of more than 30 Hz, and moreover the pulse duration of applied voltage cannot be made shorter than a limit to obtain a satisfactory display. Accordingly, the frame frequency is smaller than an upper limit, and therefore the response speed in writing and erasing information is slow and it is difficult to increase the number of picture elements.
Further, a U.S. Pat. No. 3,833,287 to Taylor et al discloses a method of driving a matrix type guest-host liquid crystal display device in which the above-mentioned holding effect is utilized to display picture images. In this method, since the device is driven based upon the hysteresis effect between the peak value of applied voltage and the relative brightness the pulse duration of applied voltage cannot be made shorter than a limit, and therefore it is hard to enhance the response speed in writing information.
U.S. Pat. No. 3,976,362 to Kawakami discloses a method of driving a matrix type liquid crystal display device with a one-line-at-a-time scanning system in which the display of picture image is controlled on the basis of the root mean square value of voltage applied to each picture element. In this driving method, the frame frequency can be made high because the picture image is displayed based upon the root mean square value of applied voltage. However, the method does not employ a phase transition liquid crystal, and therefore cannot utilize the holding effect. Accordingly, when the number of picture elements is increased, flicker appears on the picture plane, and therefore it is difficult to increase the number of picture elements.